Britain is being urged to halt plans to deport a planeload of Tamils to Sri Lanka on Wednesday amid fears they will be at risk of being detained and tortured on arrival.

Up to 50 failed asylum seekers are due to be forcibly removed from the country aboard an aircraft chartered by the UK Border Agency.

The plan has alarmed a number of NGOs, including a medical charity that treats victims of torture, which fears the British government cannot be sure that those deported will be safe in Sri Lanka.

Freedom from Torture, formerly known as the Medical Foundation for the Care of Victims of Torture, said it received 199 referrals for clinical assistance for Sri Lankans last year and a similar number this year. It said it had clinical evidence that a number of these people have been tortured in Sri Lanka since the end of the civil war in May 2009, some after being returned to the country.

The NGOs are warning there is credible evidence that torture is still taking place and anyone suspected of being linked to the Liberation Tigers of Tamil Eelam (LTTE), or Tamil Tigers, is particularly at risk.

Human Rights Watch, the New York-based NGO, said the British government "needed to be extremely careful" about returning any Tamils to Sri Lanka.

Brad Adams, the organisation's Asia director, said: "The Sri Lankan government continues to show shocking disregard for the due process rights of anyone deemed linked to the Tamil Tigers. Those detained have been tortured and 'disappeared'."

Amnesty International said it had documented evidence that failed asylum seekers had suffered torture in Sri Lanka after being removed from Australia.

In an account given to Freedom from Torture, one said: "They tortured me by removing my clothes and hitting me with burning irons. They kept me for two days and my body was all swollen. They showed me photographs of LTTE members, including my brother's picture, and asked me what he was doing now."

Keith Best, the chief executive of Freedom from Torture, has written to Damian Green, the Home Office immigration minister asking whether any arrangements are in place to ensure the safety of people removed to Sri Lanka. He said: "I know you will be mindful of the repercussions for the reputation of the UK if those who are returned then face torture."

The UK Border Agency deported 555 people to Sri Lanka last year, 235 of them failed asylum seekers.

Three months ago, Human Rights Watch warned Theresa May, the home secretary, that a group of 26 people facing deportation on a single flight were "at significant risk of persecution" in Sri Lanka. That deportation went ahead, with the government saying the Border Agency was "not aware of any difficulties" that those individuals may have subsequently faced.

An agency spokesperson said: "We only undertake returns to Sri Lanka when we are satisfied the individual has no international protection needs. The European court of human rights has ruled that not all Tamil asylum seekers require protection."

But the agency would not say what arrangements, if any, the British government had in place to monitor the treatment of those who were returned.

The agency also declined to identify the airport from which the plane is due to depart on Wednesday afternoon.

The Sri Lankan government and the Tamil Tigers were both accused of committing war crimes towards the end of the civil war.

A US diplomatic cable leaked to WikiLeaks showed that American diplomats believed the Sri Lankan president, Mahinda Rajapaksa, bore responsibility for many of the alleged crimes.

The US ambassador in Colombo, Patricia Butenis, wrote that one of the reasons there was such little progress towards a genuine Sri Lankan inquiry was that Rajapaksa and his former army commander, Sarath Fonseka, were largely responsible, and that "there are no examples we know of a regime undertaking wholesale investigations of its own troops or senior officials for war crimes while that regime or government remained in power".